1. The Field of the Invention
The present invention relates to variable frequency oscillators. More specifically, the present invention relates to methods and voltage controlled oscillator designs that compensate for errors caused by variations, from one voltage controlled oscillator to another, in the voltage that produces the nominal frequency and in the overall voltage-to-frequency transfer function.
2. Background and Related Art
Voltage controlled oscillators typically produce a nominal frequency at a given voltage. However, it is unlikely that any two randomly selected voltage controlled oscillators will produce the same nominal frequency at the same given voltage. Even though the transfer function of various voltage controlled oscillators may be substantially identical, the given voltage producing the nominal frequency is more likely to vary from one oscillator to another. In general, the operating specifications for voltage controlled oscillators include ranges of values, within expected tolerances. Voltage controlled oscillators with generous tolerances tend to be relatively less expensive, whereas voltage controlled oscillators with strict tolerances tend to be relatively more expensive.
For certain applications, generous tolerances may not present a significant problem because the applications simply do not require a high degree of precision. Other applications, however, may be relatively sensitive to even minor differences in operating characteristics. As suggested above, the traditional design approach for applications demanding a high degree of precision has been to use voltage controlled oscillators with strict tolerances. Not only are voltage controlled oscillators with strict tolerances expensive, but they also tend to be larger than voltage controlled oscillators with more generous tolerances. In part, the larger size is needed to help isolate voltage controlled oscillators with strict tolerances from environmental conditions that may impact the oscillator""s performance. For example, highly precise voltage controlled oscillators need to be isolated from changes in temperature.
Fixed oscillators generally do not suffer from the same accuracy problems as voltage controlled oscillators. Using current technology, highly precise fixed oscillators are available at a relatively inexpensive cost. To a large extent, it is the variable frequency aspect of voltage controlled oscillators that leads to precision problems. In spite of the accuracy challenges that variability introduces, voltage controlled oscillators are commonplace in a wide variety of electronic applications. For example, voltage controlled oscillators may be used in processing a digital video signal.
Frames received in a digital video signal usually are placed in a buffer of some sort prior to being displayed. On average, the rate that frames are removed from the buffer should match the rate that frames are added to the buffer; otherwise the buffer will either overflow or underflow. At any particular time, however, the add rate and the remove rate are likely to differ by some small amount. Because the add rate is fixed based on the received digital signal, the remove rate may need to be adjusted periodically. These adjustments to the remove rate should be slight because the color component of a digital video signal can be sensitive to the remove rate. The remove rate may be adjusted by a voltage controlled oscillator. Although the transfer function for most voltage controlled oscillators is relatively predictable, the voltage that produces a particular remove rate is more likely to vary from one oscillator to another.
The present invention exentends to method and voltage controlled oscillator designs that compensate for errors due to variations, from one voltage controlled oscillator to another, in the voltage that produces the nomial frequency and variations in the overall voltage-to-frequency transfer function. According to the present invenntion, for one or more voltages applied to a voltage controlled osillator, a reference frequency count is compared to a variable frequency count of the variable frequency corresponding to each of the one or more voltages that are applied to the voltage controlled oscillator. These comparisons are stored in a table that is used to determine what voltage should be applied to the voltage controlled oscillator to produce either the nominal frequency or some other frequency of interest. Generating the table may need to occur only during initialization because many voltage controlled oscillators experience little drift. However, the calibration may be repeated at other times to account for any drift that occurs.
Therefore, the present invention provides for calibrating a voltage controlled oscillator to compensate for nominal frequency and/or transfer function errors that may be present due to variations from one oscillator to another, even though the oscillators operate within specified tolerances. By dynamically determining the voltage that produces the oscillator""s nominal frequency, the present invention allows inexpensive voltage controlled oscillators having relatively generous tolerances to be used in applications that otherwise may require more precise and expensive voltage controlled oscillators. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.